My 188C sensor is inside the hull - and gives reliable depth readings at up to 40 kts. The only downside to that is that the water temperature function obviously doesn't work - but I probably wouldn't use that anyway!!
The sensor is mounted about 4 ft forward of the transom - in the front of the engine box.
I also spent quite a while setting it up to do exactly what I wanted - but being able to have so much info at a glance is really good. The colour screen is very clear. You do need to get the Blue Chart software though - the basemap isn't good enough to use as a chart.